Mosfet Circuits

Figure 1 shows a complete circuit for an emergency lamp that operates from a 12V automotive battery. The xenon flash tube requires a 250V-dc anode voltage and a 4-kV trigger pulse. To generate the 250V dc, IC1, a switching regulator controller, and T1, a standard Versa-PAC transformer, operate in the discontinuous-flyback mode. With this configuration, circuit efficiency is typically 75 to 80%. R1 and IC1's internal-sense-threshold voltage limit the peak primary current to 1.6A. The R2/R3 divider and IC1's internal 1.25V reference at the VFB Pin determine the maximum-voltage setpoint. To generate the 4-kV trigger pulse, a standard cold-cathode- fluorescent-lamp (CCFL) backlight transformer, T2, operates in the forward mode. IC2, a dual MOSFET driver, functions as a 1-Hz oscillator and a one-shot for the trigger pulse for Q2.

It does not get any easier if you want a solid state switch that is activated by the touch of a finger. Two small metal pins route voltage through the finger skin to a MOSFET switch. The circuit is great for situations where a membrane type mechanical switch is not desired.

It does not get any easier if you want a solid state switch that is activated by the touch of a finger. Two small metal pins route voltage through the finger skin to a MOSFET switch. The circuit is great for situations where a membrane type mechanical switch is not desired.

The Gate Boost SE uses a 1381, a 2N7000 MOSFET and a 2N3906 with a unique voltage doubler to increase the voltage applied to the gate of the MOSFET. Normally the 2.6V output of a 1381C is barely able to turn on the 2N7000. As a result the "on resistance" of the MOSFET is high and much power is wasted.

The circuit shows a full bridge of four MOSFETs. In forward drive, current must flow in the direction of the pale green arrow A from battery positive, through Hi1, through the motor and so through Lo2 to battery negative. In reverse drive, current must flow in the direction of the red arrow C. With the current state of ICs and N channel MOSFETs, only a fool or a masochist would use the hi-side as the main active switching element, so it is normal to apply PWM to the loside MOSFETs, and to let the Hi-side ones take a more passive role.

Modern electronic ballasts require added features, such as high-frequency preheat for filaments, nominal lamp-power control, and light-dimming capability. Because the output inverter operates in resonant mode, you can control the power or current of these ballasts simply by varying the switching frequency. To vary the frequency, you can build a simple VCO that uses a half-bridge MOSFET driver (Figure 1). You can use this VCO as the core of a controller for a supply or ballast that requires a variable frequency.

This little headphone amp from Sheldon Stokes uses a push-pull Class A MOSFET output stage, being driven by an op-amp. The output stage is included in the op-amp feedback loop, so the distortion figures are amazingly low. The power supply uses a 25 watt torroid transformer, and two adjustable regulators. It sounds very good; surprisingly un-solid state. Included with the PDF file are the schematics and the PWB layout drawings.

It is an often assumed maxim that bipolar transistors are useful for DC-DC conversion functions up to perhaps 50kHz, and for service beyond this frequency, that MOSFETs provide the only solution.

With the improvement in high power MOSFETs of late – lower gate charge, low loss gate structures, and much improved frequency capability – it has become more possible to employ these “switchmode” devices in rf generators at medium hf. The objective is to improve the ‘power density’ (W/m3) and efficiency of the equipment. The most common problem associated with this application is getting the matching right.

The I.F. amplifier is similar to the one used in the 80M receiver project . The original design has been modified by putting a couple of LED's in the source circuit of each Mosfet. The voltage drop across the LED's keeps the source voltage at about two volts. This results in a much greater AGC range. This arrangement was suggested by N6BIU. Thanks Jim.